Power amplifiers have traditionally been built as class-A and class-B for linear operation and class-C, D, E, F, and S for nonlinear operations. A primary goal of power amplifier design is to obtain a high power conversion efficiency from the DC power supply to the amplified wave output. An advantage of using the nonlinear classes is that their power-added efficiencies (PAE) tend to be higher. The PAE is a measure of how well the DC power is converted into RF or microwave power. Higher efficiencies are required in systems to preserve DC power (battery life), particularly in air borne and space borne systems, or where heat extraction is a problem, for example, in active array radar.
All of the nonlinear classes of power amplifiers generate harmonics. These harmonics must be properly terminated to prevent power from escaping the amplifier at frequencies other than the fundamental frequency. Additionally, the harmonics must be terminated at the proper phase, or the amplifier cannot operate at maximum efficiency. For example, the class-F amplifiers require the even order harmonics to be shorted and the odd order harmonics to be opened at the output of the power device. It is an extremely difficult problem to maintain the proper phase of the harmonic termination over more than the narrowest bandwidth at high frequencies, since these amplifiers generally use passive filter networks to provide the harmonic terminations.
In FIG. 2, an example of a power amplifier of the prior art is shown with harmonic termination using bandpass filters. The circuit, called a "harmonic reaction amplifier", is composed of two FETs in a configuration similar to a balanced amplifier. A second-harmonic transmission path is constructed between the FETs output terminals in each direction using bandpass filters BPF at the second-harmonic frequency (2f.sub.o, twice the fundamental frequency). The main signal output paths and second-harmonic path are designed to have matched impedance characteristics with the FETs output impedances in the fundamental frequency and in the second-harmonic frequency band, respectively. A large second-harmonic component generated at the output of each FET flows into the second-harmonic path, and is used to inject a second-harmonic component into the other FET, without reflection. Assuming the FETs have the same operating characteristics, a second-harmonic standing wave is excited along the second-harmonic path, and the second-harmonic path length is set so as to locate a voltage null point at both FET output points. This condition coincides with the output second-harmonic terminating condition in the class-F amplifier. However, this circuit is limited to the narrow bandwidth of the bandpass filters selected for a particular fundamental frequency band, in this case the 1-2 GHz microwave band, and cannot be used for a wide range of frequencies. The operation of the harmonic reaction amplifier is explained in more detail in the article "High Efficiency Microwave Harmonic Reaction Amplifier", by T. Nojima and S. Nishiki, 1988 IEEE MTT-S Digest, pp. 1007-1010.